Erlich's paper outlines the modeling of quantum chromodynamics

Traditional physics has long identified
four forces. Most people are familiar with gravity and
electromagnetism. Less well-known, except among physicists, are nuclear
interactions known as the strong force and the weak force, which is
seen in particle decay.

This is a story about the strong force but ends up involving all
four fundamental forces. The strong force is what binds protons and
neutrons together in the nuclei of atoms. A theory called quantum
chromodynamics (QCD) describes how the strong force operates.

Joshua
Erlich, assistant professor of physics at William and Mary, is one of
the authors of an article, "QCD and a Holographic Model of Hadrons,"
which outlines a proposal for modeling certain aspects of quantum
chromodynamics in five dimensions. The paper, recently published in the
journal Physical Review Letters, bolsters physicist Juan Maldacena's
connection of QCD with string theory. String theory, in effect, holds
that the four forces-gravity, electromagnetism and the strong and weak
interactions-are manifestations of the same thing.

"What Maldacena showed was that if you have a theory which looks in
many ways like quantum chromodynamics, but not quite, there's another
way of looking at it which is exactly equivalent," Erlich said. This
dualistic way of looking at things is called holography. "It's the
idea that a surface with a smaller number of dimensions might contain
all of the information to describe another kind of world which has more
dimensions in it," he explained. String theory, Erlich said, works in
10 dimensions: nine spatial dimensions plus time, although for his work
only four spatial dimensions plus time are important. Our day-to-day
existence, of course, takes place in a world of three spatial
dimensions. Add time, and ours is a four-dimensional world.

In their paper, Erlich and his co-authors outline a way to construct
a holographic twin of quantum chromodynamics. The proposed holographic
twin is a five-dimensional description of the same strong-force
interactions illustrated by quantum chromodynamics in our everyday
four-dimensional world. The five-dimensional twin is, essentially,
another window through which to view the strong interactions, although
holographic duality also has implications for string theory.

The strong and weak interactions and electromagnetism all are
governed by the same set of principles under what physicists know as
the standard model-but gravity does not fit.

"Gravity is the strangest of all the forces, meaning we understand
it the least," Erlich said. "The only quantum theory which seems to
work that includes gravity together with other interactions is string
theory." He said that the higher dimensional gravity of string theory
makes itself known through holographic duality in the form of particles
that exist in its four-dimensional twin.

"The holograph is a completely different world which has gravity in
it, and it has more dimensions than the three spatial dimensions in our
world," he said, "and they're exactly equivalent."

"It's just not obvious that these two worlds are the same," he
continued. "Anything you can calculate in one, you can calculate in
another. There's a dictionary for how to calculate things between one
world and things in another."

This dictionary, or map, between the two worlds has allowed
physicists to make a number of discoveries regarding the nature of the
strong force through a property known as chiral symmetry, Erlich said.
"This symmetry maps onto a set of interactions in this
extra-dimensional world which is dual to the strong interactions, so
now we input the physics and we ask what comes out."

What has come out, at least so far, is that predictions of the mass
of certain particles made in the five-dimensional world are accurate to
real-world measurements within 10 percent. "If you tell a string
theorist that you predicted something in the real world to 10 percent
accuracy, he'll get very excited because it's very difficult to make
physical predictions from string theory," he said.

Erlich explained that tests of the accuracy of the model make it a
good candidate for work such as calculating masses of mesons and
predicting the interaction between particles. "The physics of this is
difficult to study from the perspective of quantum chromodynamics but
easy to study from the perspective of this higher dimension," he said.
"It's really a great tool. It would be even greater if we could
understand why it works so well. If we trust it and then just follow
our nose, it makes really nice predictions."